In recent years, research is being actively conducted on electric vehicles, such as hybrid vehicles, pure electric vehicles, and fuel cell vehicles, which can replace conventional internal combustion engine vehicles, due to high oil prices, regulations to limit carbon dioxide (CO2) emissions, and the like. An electric vehicle uses a drive motor (an electric motor) as a power source, and a permanent magnet synchronous motor having high output and high efficiency, especially, an interior permanent magnet synchronous motor is usually used as the drive motor.
When braking is needed, the electric vehicle improves fuel efficiency through regenerative braking that converts kinetic energy into electrical energy. Thus, the distribution of a hydraulic braking torque using friction of a hydraulic braking system and a regenerative braking torque using a drive motor is essentially required.
A total braking torque required for normal braking may be divided as below. Here, normal braking is performed when regenerative braking energy is efficiently stored in a battery since an available storage space of the battery is sufficient.Total Braking Torque=Total Hydraulic Braking Torque+Total Regenerative Braking Torque
In addition, the total regenerative braking torque may be divided as below.Total Regenerative Braking Torque=Regenerative Braking Torque Required+Coast Regenerative Braking Torque
The regenerative braking torque required refers to a motor torque selectively applied only when a driver depresses a brake pedal, and the coast regenerative braking torque refers to a motor torque basically applied for vehicle driving comfort. In addition, as a motor torque basically applied for vehicle driving comfort, there is a creep torque.
FIG. 1 shows a graph illustrating a regenerative braking torque requirement curve, a creep torque curve, and a coast regenerative braking torque curve, in a method for controlling a motor torque in an electric vehicle according to the related art.
In FIG. 1, an X-axis represents a speed of a vehicle, a positive Y-axis represents a creep torque, a negative Y-axis represents a regenerative braking torque required, or a coast regenerative braking torque, an upward movement along the positive Y-axis represents an increase in creep torque, and a downward movement along the negative Y-axis represents an increase in regenerative braking torque required and coast regenerative braking torque.
As illustrated in FIG. 1, the required regenerative braking torque may be applied according to a predetermined regenerative braking torque requirement curve L1. According to the regenerative braking torque requirement curve L1, the required regenerative braking torque may be maintained to be a predetermined maximum regenerative braking torque T1 required, be reduced when the vehicle speed is equal to a predetermined vehicle speed V1 for reduction of the required regenerative braking torque, and be not applied when the vehicle speed is equal to a predetermined vehicle speed V2 for termination of regenerative braking torque requirement. Here, the vehicle speed V2 for termination of regenerative braking torque requirement is a vehicle speed when the required regenerative braking torque is equal to 0.
A creep torque is provided to allow a stationary vehicle with an accelerator pedal off to start forward or backward. When the stationary vehicle is in a state of a brake pedal off and the accelerator pedal off, the creep torque may allow the vehicle to slowly move forward or backward. This is called “creep driving”.
Such a creep torque may be applied according to a predetermined creep torque curve L2, as illustrated in FIG. 1. According to the creep torque curve L2, the creep torque may be maintained to be a predetermined maximum creep torque T2, be reduced when the vehicle speed is equal to a predetermined vehicle speed V3 for reduction of the creep torque, and be not applied when the vehicle speed is equal to a predetermined vehicle speed V4 for termination of creep driving. Here, the vehicle speed V4 for termination of creep driving is a vehicle speed when the creep torque is equal to 0.
The coast regenerative braking torque is provided to allow the vehicle with the accelerator pedal off to decelerate just as an internal combustion engine (ICE) works. When the vehicle speed is higher than a predetermined vehicle speed V6 for termination of coast regenerative braking in a state of the brake pedal off and the accelerator pedal off, the vehicle speed may be gradually reduced. This is called “coast driving”.
The coast regenerative braking torque may be applied according to a predetermined coast regenerative braking torque curve L3, as illustrated in FIG. 1. According to the coast regenerative braking torque curve L3, the coast regenerative braking torque may be maintained to be a predetermined maximum coast regenerative braking torque T3, be reduced when the vehicle speed is equal to a predetermined vehicle speed V5 for reduction of the coast regenerative braking torque, and be not applied when the vehicle speed is equal to the predetermined vehicle speed V6 for termination of coast regenerative braking. Here, the vehicle speed V6 for termination of coast regenerative braking is a vehicle speed when the coast regenerative braking torque is equal to 0.
FIG. 2 shows a graph illustrating a motor torque curve when braking a vehicle, in a method for controlling a motor torque in an electric vehicle according to the related art.
In FIG. 2, an X-axis represents a speed of a vehicle, a positive Y-axis represents a creep torque, a positive motor torque, or a negative Y-axis represents a regenerative braking torque required, a coast regenerative braking torque, or a negative motor torque, an upward movement along the positive Y-axis represents an increase in creep torque and positive motor torque, and a downward movement along the negative Y-axis represents an increase in regenerative braking torque required, coast regenerative braking torque, and negative motor torque.
As the vehicle speed V2 for termination of regenerative braking torque requirement is decreased, regenerative braking energy may be increased, and thus fuel efficiency may be improved. Thus, as illustrated in FIG. 1, there is a section in which the vehicle speed V2 for termination of regenerative braking torque requirement is lower than the vehicle speed V4 for termination of creep driving. Then, an overlapping region in which a positive torque command and a negative torque command coexist may appear in a section between the vehicle speed V2 for termination of regenerative braking torque requirement and the vehicle speed V4 for termination of creep driving.
In general, when such an overlapping region is generated, the regenerative braking torque requirement may be prioritized for safety. As illustrated in FIG. 2, when the vehicle is braked, if the vehicle speed is between the vehicle speed V2 for termination of regenerative braking torque requirement and the vehicle speed V4 for termination of creep driving, the negative motor torque corresponding to the required regenerative braking torque may be output, and if the vehicle speed is lower than the vehicle speed V2 for termination of regenerative braking torque requirement, the positive motor torque corresponding to the creep torque may be output. In other words, there is a section in which the motor torque is suddenly changed from negative torque to positive torque on the basis of the vehicle speed V2 for termination of regenerative braking torque requirement. Thus, due to a sudden change of the motor torque occurring when the required regenerative braking torque is changed to the creep torque in the conventional method for controlling a motor torque in an electric vehicle, a shock may occur in a drive system of the vehicle, causing a reduction in ride comfort.
Meanwhile, as illustrated in FIG. 1, the vehicle speed V5 for reduction of the coast regenerative braking torque and the vehicle speed V6 for termination of coast regenerative braking may be higher than the vehicle speed V1 for reduction of the required regenerative braking torque and the vehicle speed V2 for termination of regenerative braking torque requirement. In other words, the vehicle speed V5 for reduction of the coast regenerative braking torque and the vehicle speed V6 for termination of coast regenerative braking differ from the vehicle speed V1 for reduction of the required regenerative braking torque and the vehicle speed V2 for termination of regenerative braking torque requirement. Thus, as illustrated in FIG. 2, inflection points P1 and P2 appear at particular points of the motor torque curve L3 corresponding to the vehicle speed V5 for reduction of the coast regenerative braking torque and the vehicle speed V6 for termination of coast regenerative braking. In the conventional method for controlling a motor torque in an electric vehicle, since the required regenerative braking torque and the coast regenerative braking torque are unsynchronized, a difference therebetween may cause a reduction in ride comfort.